Just from the concept (my cuz only let me use it a few times), Game Boy Player is perhaps the best.
It gives you access to 99% of the GB, GBC, and GBA libraries.
Super Game Boy also deserves some love for doing it first, but you obviously can't use any of the GBA library and none of the Game Boy Color clear cartridges that don't work on Game Boy.
And that's it.
Lifetime Sales Predictions
Switch: 161 million (was 73 million, then 96 million, then 113 million, then 125 million, then 144 million, then 151 million, then 156 million)
PS5: 115 million (was 105 million) Xbox Series S/X: 48 million (was 60 million, then 67 million, then 57 million)
PS4: 120 mil (was 100 then 130 million, then 122 million) Xbox One: 51 mil (was 50 then 55 mil)
3DS: 75.5 mil (was 73, then 77 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima